1. Field of the Invention
The present invention relates to a compact dual band antenna that is capable of transmitting and receiving signal waves of two different frequency bands, and is ideally built in an in-car communication device or the like.
2. Description of the Related Art
FIG. 6 shows a conventionally known antenna device as one of the abovementioned type of dual band antennas. In this antenna device, a radiating conductor formed by connecting two types of meander lines having different pitches is provided on a surface of a substrate (refer to, for example, pages 3 to 4 and FIG. 1 in Japanese Unexamined Patent Application Publication No. 2001-68917).
In a dual band antenna 1 shown in FIG. 6, a radiating conductor 4 formed of copper foil or the like is patterned on a surface of a dielectric substrate 3 vertically provided on a grounding conductor plate 2. The radiating conductor 4 combines a first radiating conductor portion 4a formed to extend in a meander-shape at a relatively wide pitch from a vicinity of a feeding point and a second radiating conductor portion 4b formed to extend in a meander-shape at a relatively narrow pitch from a distal end of the first radiating conductor portion 4a. 
In the dual band antenna 1 constructed as described above, the entire radiating conductor 4 from the first radiating conductor portion 4a to the second radiating conductor portion 4b resonates at a first frequency f1 by supplying first high-frequency power to a feeding point of the radiating conductor 4 through a feeder line, such as a coaxial cable. In addition, only the first radiating conductor portion 4a resonates at a second frequency f2, which is higher than the first frequency f1, by supplying second high-frequency power to the feeding point. In other words, it is hard for a high frequency current of a higher frequency to pass through the meander line with a narrow pitch, namely, the second radiating conductor portion 4b, thus making it possible to actuate only the first radiating conductor portion 4a as a radiating element in response to the second frequency f2. The radiating conductor 4 formed in the meander shape allows the height of the dual band antenna 1 to be considerably reduced at the same electrical length, as compared with a radiating conductor that merely extends in a linear fashion. Thus, the meander shape arrangement is advantageous in making an entire dual band antenna smaller and shorter.
In the conventional dual band antenna 1 shown in FIG. 6, if the meandering pitch or the spacing of the meandering portions of the radiating conductor 4 is excessively narrow, then high-order modes are generated when supplying power to the radiating conductor 4. To avoid this problem, the radiating conductor 4 is formed in a narrower strip to facilitate a reduction in height. Making the radiating conductor 4 narrower, however, results in a narrower resonance frequency band. Therefore, to restrain degradation of the antenna performance, the radiating conductor 4 is designed such that the strip width is large enough while the meander pitch not excessively narrow. Thus, if the two different types of radiating conductor portions 4a and 4b having different meander pitches are connected in series, as in the case of the conventional dual band antenna 1, then the radiating conductor 4 is naturally lengthy, making it difficult to reduce the height of the entire antenna.